The goal is to use the rat as a model for studying factors that may influence the establishment of methanogenesis in humans and the significance of methanogenesis to the physiology of the host. Methanogenic bacteria of Wistar and other strains of rats will be isolated and characterized. Methanogens and total viable anaerobes will be concurrently enumeratd in rats on low, normal and high fiber diets and the influence or the diets on the production of CH4, H2 and volatile fatty acids will be measured. Establishment of methanogenic bacteria as a function of age an sex will be investigated. Development of antibodies to autochthonous methanogens will be studied and rats will be immunized with autochthonous strains to determine if antibodies cause elimination of methanogens from the intestine. Non-methanogenic rat strains will be manipulated by varying diets and inoculation with flora from methanogenic strains to see if methanogenesis can be established. Experiments designed to measure the relative contribution of dietary and endogenous substrates to large intestine microbial fermentation are proposed. These involve the use of a basal synthetic diet with nutrients that are completely absorbed in the small intestine. Total CH4 and H2 production as well as the rate of dilution of labeled acetate added to the cecum will be used to measure the contribution of endogenous substrates to fermentation. Identical measurements will be made after additions of substrates that escape monogastric digestion to determine their addtional contribution ot fermentation. The results should indicate why CH4 is a product of large intestine fermentation in 1/3 to 1/2 of normal human adults and not in the rest of the population. Evaluation of the contribution of endogenous substrates to fermentation may provide a basis for using measurements of H2 and CH4 production to evaluate differences in the contribution of endogenous substrates to fermentation in different individuals.